There has been a long felt need for a motor or actuator which has high torque, high stiffness, zero backlash, is compact and creates a reduced or minimal magnetic field. There also has been a need for such a motor which is capable of operating in a fine stepping mode. Such a device has particular application in mechanisms used in outer space as well as for robotics, precise control of telescopes or other precision instruments. Although many attempts have been made to utilize the unique capabilities of piezoelectric devices, none have been utilized to create an actuator or motor which incorporates all of the above-mentioned features.
Meissner U.S. Pat. No. 1,804,838 uses oscillations of a piezoelectric crystal to vibrate a disk into rotary motion. The torque output is quite small and the step size is nonrepeatable.
Duff et al. U.S. Pat. No. 3,315,103 uses an electrostrictive substance to produce angular deflections of the tip of a shaft. The actuator is subdivided in such a way as to allow digital control. This actuator does not produce rotary motion.
Vishnevsky et al. U.S. Pat. No. 4,019,073 describes a whole family of piezoelectric motor designs with vibrating rotors and/or stators. In all cases, however, the rotors and stators are held in contact by relatively light spring forces so that the torque output is quite low.
Toda et al. U.S. Pat. No. 4,339,682 and Osaka et al. U.S. Pat. No. 4,399,386 each uses a bimorph configuration of piezoactuator to generate a low force friction drive.
Tojo et al. U.S. Pat. No. 4,455,501 uses piezoactuators as both a locking or clamping device and a driving device. By properly timing the locking and unlocking with the extend-retract cycle of the drive actuators, the output shaft is made to rotate. This type of device should produce higher torques than the previously described devices, as well as repeatable step sizes. Mori U.S. Pat. No. 4,468,583 is a different configuration of the Tojo et al. patent.
Igashira et al. U.S. Pat. No. 4,471,256 discloses another piezoelectric actuator using a multitude of piezoelectric disks to produce a high-force device with a "large stroke", probably a few thousandths of an inch, with a "low" drive voltage. The voltage is probably 500 V or less, rather than the much higher voltage required for a solid stack of this length.
Sashida U.S. Pat. No. 4,562,374 includes a piezoelectric or electrostrictive driver which produces a standing or traveling surface wave in an elastic element. Since the wave crests tend to rotate in clockwise or counterclockwise ellipses, any movable surface held against the wave crests will be moved along in a rotary or linear mode depending on the configuration of the motor. The normal force is low and the output torque is in the inch-ounce range. This patent shows a version of a piezo motor using a hypocyloid or harmonic type of drive. The method of holding the driver or roller against each other is of the low force variety as it depends on a bending motion of the roller shaft.
Glett et al. U.S. Pat. No. 4,636,679 is directed to a clutch; however, it does show the use of piezoelectric stack to produce large forces.
Rodloff et al. U.S. Pat. No. 4,639,630 uses a piezoceramic in a stacked arrangement to adjust mirrors for a laser device.
A common difficulty with most prior art devices is that they must run at resonant frequencies, which are usually in the ultrasonic range, which is only practical for motors of small size and torque.